Stereoscopic projection system using tunable light emitters

ABSTRACT

A stereoscopic digital projection system that projects stereoscopic images including first-eye images and second-eye images. The system includes one or more narrow-band, solid-state, tunable light emitters, each being controllable to alternately provide emitted light in a first state in a corresponding first spectral band and emitted light in a second state in a corresponding second spectral band. An image forming system including at least one spatial light modulator is used to form modulated images by modulating light from the tunable lights emitters. A controller synchronously controls the state of the tunable light emitters and the spatial light modulator pixels, wherein the spatial light modulator pixels are controlled responsive to first-eye image data when the tunable light emitters are in the first state and are controlled responsive to second-eye image data when the tunable light emitters are in the second state.

CROSS-REFERENCE TO RELATED APPLICATIONS

Reference is made to commonly assigned, co-pending U.S. patentapplication Ser. No. ______ (Docket K000613), entitled: “Stereoscopicprojector using spectrally-adjacent color bands”, by Silverstein et al.;to commonly assigned, co-pending U.S. patent application Ser. No. ______(Docket K000614), entitled: “Stereoscopic projector using scrollingcolor bands”, by Silverstein et al.; to commonly assigned, co-pendingU.S. patent application Ser. No. ______ (Docket K0000804), entitled:“Stereoscopic glasses using dichroic and absorptive layers”, bySilverstein et al.; to commonly assigned, co-pending U.S. patentapplication Ser. No. ______ (Docket K000805), entitled: “Spectralstereoscopic projection system”, by Silverstein et al.; to commonlyassigned, co-pending U.S. patent application Ser. No. ______ (DocketK000817), entitled: “Filter glasses for spectral stereoscopic projectionsystem”, by Silverstein et al.; and to commonly assigned, co-pendingU.S. patent application Ser. No. ______ (Docket K000833), entitled:“Stereoscopic glasses using tilted filters”, by Silverstein et al., eachof which is incorporated herein by reference.

FIELD OF THE INVENTION

This invention relates to a stereoscopic digital projection system thatuses spectrally-adjacent light sources to form left-eye and right-eyeimages, and more particularly to a stereoscopic digital projectionsystem that provides the spectrally-adjacent light sources using tunablelight emitters.

BACKGROUND OF THE INVENTION

In order to be considered as suitable replacements for conventional filmprojectors, digital projection systems must meet demanding requirementsfor image quality. This is particularly true for multicolor cinematicprojection systems. Competitive digital projection alternatives toconventional cinematic-quality projectors must meet high standards ofperformance, providing high resolution, wide color gamut, highbrightness, and frame-sequential contrast ratios exceeding 2,000:1.

Stereoscopic projection is a growing area of special interest for themotion picture industry. Three-dimensional (3-D) images or perceivedstereoscopic content offer consumers an enhanced visual experience,particularly in large venues. Conventional stereoscopic systems havebeen implemented using film, in which two sets of films and projectorssimultaneously project orthogonal polarizations, one for each eye,termed a “left-eye image” and a “right-eye image” in the context of thepresent disclosure. Audience members wear corresponding orthogonallypolarized glasses that block one polarized light image for each eyewhile transmitting the orthogonal polarized light image.

In the ongoing transition of the motion picture industry to digitalimaging, some vendors, such as IMAX, have continued to utilize atwo-projection system to provide a high quality stereo image. Morerecently, however, conventional digital projectors have been modified toenable 3D projection.

Conventional methods for forming stereoscopic images from these digitalprojectors have used one of two primary techniques for distinguishingleft- and right-eye images. One technique, utilized by DolbyLaboratories, for example, uses spectral or color space separation. Themethod used is similar to that described in U.S. Pat. No. 7,832,869,entitled “Method and device for performing stereoscopic image displaybased on color selective filters” to Maximus et al., wherein color spaceseparation is used to distinguish between the left-eye and right-eyeimage content. The image for each eye is projected using primary Red,Green, and Blue component colors, but the precise Red, Green, and Bluewavelengths that are used differ between left- and right-eye images. Toachieve this separation, filters are utilized in the white lightillumination system to momentarily block out portions of each of theprimary colors for a portion of the frame time. For example, for theleft eye, the lower wavelength spectrum of Red, Blue, and Green (RGB)would be blocked for a period of time. This would be followed byblocking the higher wavelength spectrum of Red, Blue, and Green (RGB)for the other eye. The appropriate color adjusted stereo content that isassociated with each eye is presented to each spatial light modulatorfor the eye. The viewer wears viewing glasses with a correspondingfilter set that similarly transmits only one of the two 3-color (RGB)spectral sets to each eye.

A second approach utilizes polarized light. One method disclosed in U.S.Pat. No. 6,793,341 to Svardal et al., utilizes each of two orthogonalpolarization states delivered to two separate spatial light modulators.Polarized light from both modulators is then projected simultaneously.The viewer wears polarized glasses with polarization transmission axesfor left and right eyes orthogonally oriented with respect to eachother.

There are advantages and drawbacks with each approach. Spectralseparation solutions, for example, are advantaged by being more readilyusable with less expensive display screens. With spectral separation,polarization properties of the modulator or associated optics do notsignificantly affect performance. However, the needed filter glasseshave been expensive and image quality is reduced by factors such asangular shift, head motion, and tilt. Expensive filter glasses are alsosubject to scratch damage and theft. Promising developments in filterglass design, including the use of layered optical films produced bynon-evaporative means by 3M Corp, can help to address the cost problemand make spectral separation techniques more cost-effective.

Another drawback of the spectral separation approach relates todifficulties in adjustment of the color space and significant light lossdue to filtering, leading to either a higher required lamp output orreduced image brightness. Filter losses have been addressed in U.S.Patent Application Publication 2009/0153752 to Silverstein, entitled“Projector using independent multiple wavelength light sources” whereinindependent spectrally-adjacent sources are combined by a beamsplitterto be efficiently directed to a spatial light modulator. Onedisadvantage of this approach is that these light sources are onlyutilized approximately half of the time, as the modulator can onlyprovide one eye image in time. While the light sources will likely havea longer life, the initial cost of the display is increase by the costrequirement of two sets of independent sources.

With polarization for separating the left- and right-eye images, lightcan be used more efficiently. U.S. Pat. No. 7,891,816 to Silverstein etal., entitled “Stereo projection using polarized solid state lightsources,” and U.S. Pat. No. 8,016,422 to Silverstein et al., entitled“Etendue maintaining polarization switching system and related methods,”describe projection system configurations that fully utilize the lightsource for both polarization states. However, polarization techniquesare disadvantaged by the additional cost and sensitivity of polarizationmaintaining screens, which typically utilize a structured metalliccoating. These coatings are high gain, which improves on axis viewing,but are poor for off axis viewing. Furthermore, the specular reflectionswith this method can be troubling for some viewers. This effect isfurther exacerbated when using coherent light, as it leads to higherlevels of viewer perceived speckle. Projectors using polarized light aretypically more costly due to the difficulty of maintaining highpolarization control through high angle optics as well as being moresensitive to dirt and defects. Therefore any gains in efficiency can besomewhat offset by other problems.

A continuing problem with illumination efficiency relates to etendue or,similarly, to the Lagrange invariant. As is well known in the opticalarts, etendue relates to the amount of light that can be handled by anoptical system. Potentially, the larger the etendue, the brighter theimage. Numerically, etendue is proportional to the product of twofactors, namely the image area and the numerical aperture. In terms ofthe simplified optical system represented in FIG. 1 having light emitter12, optics 18, and a spatial light modulator 20, the etendue of thelight source is a product of the light source area A1 and its outputangle θ1. Likewise, the etendue of the spatial light modulator 20 equalto the product of the modulator area A2 and its acceptance angle θ2. Forincreased brightness, it is desirable to provide as much light aspossible from the area of light source 12. As a general principle, theoptical design is advantaged when the etendue at the light emitter 12 ismost closely matched to the etendue at the spatial light modulator 20.

Increasing the numerical aperture, for example, increases the etendue sothat the optical system captures more light. Similarly, increasing thelight source size, so that light originates over a larger area,increases etendue. In order to utilize an increased etendue on theillumination side, the etendue of the spatial light modulator 20 must begreater than or equal to that of the light source 12. Typically,however, the larger the spatial light modulator 20, the more costly itwill be. This is especially true when using devices such as LCOS and DLPcomponents, where the silicon substrate and defect potential increasewith size. As a general rule, increased etendue results in a morecomplex and costly optical design.

Efficiency improves when the etendue of the light source is well-matchedto the etendue of the spatial light modulator. Poorly matched etenduemeans that the optical system is either light-starved, unable to providesufficient light to the spatial light modulators, or inefficient,effectively discarding a substantial portion of the light that isgenerated for modulation.

Solid-state lasers promise improvements in etendue, longevity, andoverall spectral and brightness stability. Recently, devices such asVCSEL (Vertical Cavity Surface-Emitting Laser) laser arrays have beencommercialized and show some promise, when combined in various ways, aspotential light sources for digital cinema projection. However,brightness itself is not yet high enough; the combined light from asmany as 9 individual arrays is needed in order to provide the necessarybrightness for each color.

Laser arrays of particular interest for projection applications arevarious types of VCSEL arrays, including VECSEL (Vertical ExtendedCavity Surface-Emitting Laser) and NECSEL (Novalux Extended CavitySurface-Emitting Laser) devices from Novalux, Sunnyvale, Calif.

However, even with improvements in laser technology and in filterpreparation and cost, there is considerable room for improvement inmethods of stereoscopic imaging projection. Conventional solutions thatuse spectral separation of left- and right-eye images are typicallylight-starved, since at most only half of the light that is generated isavailable for each eye. Thus, there is a need for a stereoscopic imagingsolution that offers increased optical efficiencies with decreasedoperational and equipment costs.

SUMMARY OF THE INVENTION

The present invention represents a stereoscopic digital projectionsystem that projects a stereoscopic image including a first-eye imageand a second-eye image onto a display surface, comprising:

one or more narrow-band, solid-state, tunable light emitters, eachcontrollable to alternately provide emitted light in a first statehaving a corresponding first spectral band with a respective firstcentral wavelength and emitted light in a second state having acorresponding second spectral band with a respective second centralwavelength, the first and second spectral bands being substantiallynon-overlapping;

an image forming system including at least one spatial light modulatorfor forming modulated images by modulating light from the tunable lightsemitter, each spatial light modulator having an array of controllablespatial light modulator pixels;

a controller that synchronously controls the state of the tunable lightemitters and the spatial light modulator pixels, wherein the spatiallight modulator pixels are controlled responsive to image data for thefirst-eye image when the tunable light emitters are in the first statethereby providing a first-eye modulated image and the spatial lightmodulator pixels are controlled responsive to image data for thesecond-eye image when the tunable light emitters are in the second statethereby providing a second-eye modulated image; and

projection optics for delivering the first-eye modulated image and thesecond-eye modulated image to a display surface.

This invention has the advantage that the use of tunable light-emittersreduces the system cost by reducing the number of required components.

It has the additional advantage that the tunable light-emitters can beused to provide speckle reduction by varying the wavelength of theemitted light within the associated spectral bands.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a representative diagram showing factors in etenduecalculation for an optical system;

FIG. 2 is a schematic block diagram that shows a stereoscopic projectionapparatus that uses spectral separation for left- and right-eye images;

FIG. 3A is a schematic diagram showing a prior art color scrollingsequence;

FIG. 3B is a schematic diagram showing a single-channel color scrollingsequence using spectrally-adjacent bands of color according to anembodiment of the present invention;

FIG. 4A is a schematic diagram that shows parts of a single colorchannel in a stereoscopic digital projection system that uses a singlebeam scanner to provide two spectrally-adjacent bands of color;

FIG. 4B is a schematic diagram that shows parts of a single colorchannel in a stereoscopic digital projection system that uses a separatebeam scanner to provide each spectrally-adjacent band of color;

FIG. 5 is a schematic diagram showing a stereoscopic digital projectionsystem having three color channels, each using the configuration of FIG.4A;

FIG. 6A is a schematic diagram that shows the use of a rotating prismfor scanning a single band of color;

FIG. 6B is a schematic diagram that shows the use of a rotating prismfor scanning two bands of color;

FIG. 6C is a schematic diagram showing another configuration for using arotating prism for scanning two bands of color;

FIG. 7A is a schematic diagram that shows uniformizing optics includingtwo lenslet arrays;

FIG. 7B is a schematic diagram that shows uniformizing optics includingtwo integrating bars;

FIG. 8 is a schematic diagram showing a beam scanning configurationaccording to an embodiment of the present invention;

FIG. 9 is a schematic diagram of a stereoscopic color scrolling digitalprojection system having three color channels and using combining opticsfor arrays of solid-state light emitters;

FIG. 10 is a schematic diagram of a stereoscopic color scrolling digitalprojection system having three color channels according to an alternateembodiment using two spatial light modulators;

FIG. 11A is a graph that shows spectral bands for stereoscopicprojection using spectral separation in an interleaved arrangement;

FIG. 11B is a graph that shows spectral bands for stereoscopicprojection using spectral separation in an alternate non-interleavedarrangement;

FIG. 12A is a graph that shows spectral transmittances for right-eye andleft-eye filters for use with the interleaved spectral band arrangementof FIG. 3A;

FIG. 12B is a graph that shows spectral transmittances for right-eye andleft-eye filters for use with the non-interleaved spectral bandarrangement of FIG. 3B;

FIG. 13 is a graph illustrating the origin of crosstalk in awavelength-based stereoscopic imaging system;

FIG. 14 is a graph illustrating the angular dependent of the spectraltransmission characteristics for left-eye and right-eye eye filters usedin commercially available filter glasses;

FIGS. 15A and 15B are cross-section diagrams showing embodiments ofright-eye filters having a dichroic filter stack;

FIG. 16A is a graph showing spectral transmittance characteristics foran example right-eye filter using a dichroic filter stack;

FIG. 16B is a graph showing transmitted light provided by the right-eyefilter of FIG. 16A;

FIG. 16C is a graph showing spectral reflectance characteristics for theright-eye filter of FIG. 16A;

FIG. 16D is a graph showing reflected light provided by the right-eyefilter of FIG. 16A;

FIGS. 17A-17D are cross-section diagrams showing embodiments ofright-eye filters having a dichroic filter stack and one or moreabsorptive filter layers;

FIG. 18 is a graph showing spectral transmittance characteristics for anexample dichroic filter stack and an example absorptive filter layerappropriate for use in a right-eye filter;

FIG. 19A is a graph showing spectral transmittance characteristics foran example hybrid right-eye filter that combines a dichroic filter stackand an absorptive filter layer;

FIG. 19B is a graph showing transmitted light provided by the hybridright-eye filter of FIG. 19A;

FIG. 19C is a graph showing spectral reflectance characteristics for thehybrid right-eye filter of FIG. 19A;

FIG. 19D is a graph showing reflected light provided by the hybridright-eye filter of FIG. 19A;

FIG. 20A is a schematic diagram showing a path of light reflected lightfrom filter glasses for two observers having heads at the same height;

FIG. 20A is a schematic diagram showing a path of light reflected lightfrom filter glasses for two observers having heads at different heights;

FIG. 21A is a side view showing filter glasses with tilted filterelements;

FIG. 21B is a perspective view showing filter glasses with tilted filterelements;

FIG. 21C is a side view showing filter glasses with a hinge foradjusting a tilt angle for tilted filter elements;

FIG. 22 is a side view that showing observers wearing filter glasseswith tilted filter elements;

FIG. 23 is a schematic diagram showing a path of light reflected lightfrom filter glasses with tilted filter elements for two observers havingheads at different heights;

FIG. 24 is a schematic diagram showing one color channel of astereoscopic imaging system for forming right-eye and left-eye imagesusing tunable light emitters; and

FIG. 25 is a schematic diagram showing a color stereoscopic imagingsystem for forming right-eye and left-eye images using tunable lightemitters.

DETAILED DESCRIPTION OF THE INVENTION

The invention is inclusive of combinations of the embodiments describedherein. References to “a particular embodiment” and the like refer tofeatures that are present in at least one embodiment of the invention.Separate references to “an embodiment” or “particular embodiments” orthe like do not necessarily refer to the same embodiment or embodiments;however, such embodiments are not mutually exclusive, unless soindicated or as are readily apparent to one of skill in the art. The useof singular or plural in referring to the “method” or “methods” and thelike is not limiting. It should be noted that, unless otherwiseexplicitly noted or required by context, the word “or” is used in thisdisclosure in a non-exclusive sense.

The present description is directed in particular to elements formingpart of, or cooperating more directly with, apparatus in accordance withthe invention. It is to be understood that elements not specificallyshown or described may take various forms well known to those skilled inthe art.

Figures shown and described herein are provided to illustrate principlesof operation according to the present invention and are not drawn withintent to show actual size or scale. Because of the relative dimensionsof the component parts for the laser array of the present invention,some exaggeration is necessary in order to emphasize basic structure,shape, and principles of operation. In addition, various components suchas those used to position and mount optical components, for example, arenot shown in order to better show and describe components that relatemore closely to embodiments of the present invention.

Where they are used, the terms “first”, “second”, and so on, do notnecessarily denote any ordinal or priority relation, but may be simplyused to more clearly distinguish one element from another.

The terms “color” and “wavelength band” and “spectral band” aregenerally synonymous as used in the context of the present disclosure.For example, a laser or other solid-state light source is referred to byits general color spectrum, such as red, rather than by its peak outputwavelength (such as 635 nm) or its spectral band (such as 630-640 nm).In the context of the present disclosure, different spectral bands areconsidered to be essentially non-overlapping.

The terms “viewer” and “observer” are used equivalently to refer to aperson viewing the stereoscopic display of the present invention. Theterm “left-eye image” refers to the image formed for viewing by the lefteye of the observer. Correspondingly, the term “right-eye image” refersto the image formed for viewing by the right eye of the observer.

Embodiments of the present invention address the need for improvedbrightness in a stereoscopic viewing system using independent adjacentspectral sources.

In the context of the present invention, the terms “transmission band”and “pass band” are considered to be equivalent.

In the context of the present invention, the term “spectrally-adjacent”relates to nearby spectral bands within the general color spectrum thatare used for the component colors that form a color image, typicallyred, green, blue, and possibly including a fourth color and otheradditional colors. The corresponding spectrally-adjacent colors for eachcomponent color lie in the same color spectrum, but have differentwavelength ranges for left- and right-eye images such that the spectralbands are substantially non-overlapping with respect to wavelength.

FIG. 2 is a schematic block diagram of an image-forming system thatillustrates some of the major components of a stereoscopic digitalprojection system 110 including a projector apparatus 120 that usesspectral separation for forming left-eye and right-eye images on aviewing screen or other type of display surface 72. A first set ofright-eye light emitters 12R emit light in a first red spectral band R1,a first green spectral band G1, and a first blue spectral B1. Theright-eye light emitters 12R are used to form a right-eye image forviewing by an observer's right eye. Similarly, a second set of left-eyelight emitters 12L emit light in a second red spectral band R2, a secondgreen spectral band G2, and a second blue spectral B2. The left-eyelight emitters 12L are used to form a left-eye image for viewing by anobserver's left eye.

In a preferred embodiment, the spectral bands associated the left-eyelight emitters 12L and the right-eye light emitters 12R are allsubstantially non-overlapping with each other so that filter glasses 74can be used to effectively separate the light provided by the left-eyelight emitters 12L from the light provided by the right-eye lightemitters 12R. By substantially non-overlapping we mean that the spectralpower from one spectral band is negligible for any wavelength whereanother spectral band is non-negligible. Acceptable results cansometimes be obtained even when there is some small level of overlapbetween the spectral bands. One criterion that can be used in practiceis that less than 5% of the light from one of the spectral bands shouldoverlap with the other spectral band.

The filter glasses 74 include a left-eye filter 76L and a right-eyefilter 76R, together with a frame 62 into which the left-eye filter 76Land the right-eye filter 76R are filtered are mounted. The frame 62 isadapted to position the right-eye filter 76R in front of the observer'sright eye and to position the left-eye filter 76L in front of theobserver's left eye. The right-eye filter 76R has spectral transmissioncharacteristics that are adapted to transmit the light in the R1, G1 andB1 spectral bands from the right-eye light emitters 12R and to block(i.e., absorb or reflect) the light in the R2, G2 and B2 spectral bandsfrom the left-eye light emitters 12L. Likewise, the left-eye filter 76Lhas spectral transmission characteristics that are adapted to transmitthe light in the R2, G2 and B2 spectral bands from the left-eye lightemitters 12L and to block the light in the R1, G1 and B1 spectral bandsfrom the right-eye light emitters 12R.

Projector apparatus 120 can have two separate projector devices, onewith color channels intended to serve a left-eye imaging path thatprojects light from the left-eye light emitters 12L and the other toserve a right-eye imaging path that projects light from the right-eyelight emitters 12R. However, many designs combine the left-eye andright-eye imaging functions into a single projector, such as to takeadvantage of inherent alignment characteristics and to reduce the costassociated with components such as projection lenses. Subsequentdescription in this disclosure gives detailed information on one type ofprojector that combines left-eye and right-eye imaging paths using colorscrolling. It can be appreciated by those skilled in the imageprojection arts that there are also other methods available forcombining stereoscopic left-eye and right-eye images. Embodiments of thepresent invention can be used with any of a number of types ofstereoscopic projection systems that utilize spectral separationtechniques.

The schematic diagram of FIG. 3A shows how a color scrolling sequencecan be used to provide a color image from component red (R), green (G),and blue (B) light in conventional practice, for a projection apparatusthat is not stereoscopic. A series of image frames 28 a, 28 b, 28 c, 28d, and 28 e are shown as they are arranged at different times. Eachframe has three bands of light 34 r, 34 g, and 34 b having red, greenand blue color components, respectively, that are scanned across imageregion 32, moving in a vertical direction in the example shown. As aband is scrolled off the bottom of the image frame, it is scrolled intothe top of the image frame so that ⅓ of the image frame is covered byeach of the color components at any given time.

A vertical scrolling motion is generally preferred because horizontalscrolling can be impacted by side to side movement of the viewer wherebythe color bands may become perceptible. This is often referred to as a“rainbow effect.” The bands of light in this sequence can be fromillumination components, scanned onto the spatial light modulator or maybe imaged light from the spatial light modulator. The scanning action iscyclic, recurring at an imperceptible rate for the viewer, at a rate ofmany times per second (e.g., 144 Hz). As can be seen from this sequence,each image frame 28 a, 28 b, 28 c, 28 d and 28 e has each of the threecomponent colors scanned over a different image region. In the imagethat is formed using this sequence, each frame has red, green, and blueimage content, in the respective bands of light 34 r, 34 g, and 34 b.

It can be readily appreciated that the color scrolling scheme of FIG.3A, while usable for non-stereoscopic color imaging, presentsdifficulties for stereoscopic color imaging systems. Providingstereoscopic color requires the scrolling of six different spectralbands, two for each of the component colors. Each source has its ownetendue associated with it. Illuminating a single chip with sixdifferent sources, each also requiring a gap between them to preventcrosstalk and allowing for chip transition time from each of the colordata associated with the particular color would quickly utilize theavailable etendue or require optically fast lenses. While this isfeasible, it is undesirable, since projector brightness is severelyconstrained and cost of the optics quickly rises with such anarrangement.

To help improve image quality and deliver higher brightness,cinematic-quality projection systems for non-stereoscopic imaging oftenemploy separate color channels for each color, typically providing eachof a red, green, and blue color channel. A spatial light modulator isprovided in each color channel. This arrangement enables the opticaldesign to optimize the design and features of components, such asfilters and coatings, for example, to improve their performance forlight of the respective wavelengths.

FIG. 3B shows a color scanning arrangement for a stereoscopic projectionsystem according to an exemplary embodiment of the present invention. Inthis configuration, spectrally-adjacent spectral bands within a singlecomponent color spectrum are scrolled across the image region 32, ratherthan bands corresponding to the different color components as in thearrangement of FIG. 2. In this example, spectrally-adjacent red spectralbands R1 and R2 are scrolled, as bands of light 36 a and 36 b, acrossimage frames 38 a, 38 b, 38 c, 38 d, and 38 e according to an embodimentof the present invention. The R1 spectral band is used to provide theleft-eye image and the R2 spectral band is used to provide the right-eyeimage for the projected stereoscopic image. Similar spectral scrollingmechanisms are provided for each color channel of the stereoscopicimage, as will subsequently be described in more detail. Further bymaintaining the light of the same color within its own color channel,the optical coatings for the optical components associated with aparticular color component can continue to be optimized for therespective color component.

The schematic diagrams of FIGS. 4A and 4B show parts of a red colorchannel 40r for color scrolling spectrally-adjacent colors in a singlecolor channel, compatible with an embodiment of the present invention. Alight source 42 a emits a beam of light in the R1 spectral band, andanother light source 42 b emits a beam of light in the R2 spectral band.Illumination optics 90 provide substantially uniform bands of light ontospatial light modulator 60 for modulation in each of the twospectrally-adjacent spectral bands. Beam scanning optics 92 including abeam scanner 50 provide the cyclical scrolling of the bands of light. Itwill be recognized that the illumination optics 90 can include multiplelens 48, some of which may be positioned between the uniformizing optics44 and the beam scanning optics 92, with others being positioned betweenthe beam scanning optics 92 and the spatial light modulator 60. In apreferred embodiment, the illumination optics 90 image an output face ofthe uniformizing optics 44 onto the spatial light modulator 60, therebyproviding the uniform bands of light. An advantage of this approach isthat the light sources 42 a and 42 b can be continuously on duringprojection, providing increased light output over other stereoscopicprojection methods.

In the configuration of FIG. 4A, a beam combiner 46 combines the lightbeams from the light sources 42 a and 42 b onto parallel optical axesand directs the spatially-adjacent light beams into uniformizing optics44, such as one or more lenslet arrays or uniformizing bars, to providesubstantially uniform spatially-adjacent light beams. A beam scanner 50then cyclically scrolls the combined uniformized light and directs thescrolled combined light beam onto the spatial light modulator 60 throughthe illumination optics 90, which provide for beam imaging, shaping andconditioning. In FIG. 4A, the illumination optics 90 are represented aslens 48; however in various embodiments the illumination optics 90 caninclude different (or multiple) optical components. The beam separationrequired to prevent crosstalk between the bands of light may be providedby use of spatial or angular separation of the incoming beams of lightto beam scanner 50. In the event that differing angles are utilized, itis generally desired that another element, such as a dichroic beamcombiner, be provided downstream of the beam scanner 50 to return thescanned beams of light onto parallel optical axes.

The spatial light modulator 60 forms an image frame 38 havingcorresponding bands of light 36 a and 36 b. The bands of light 36 a and36 b are cyclically scrolled as described previously. The spatial lightmodulator 60 has an array of pixels that can be individually modulatedaccording to image data to provide imaging light. The spatial lightmodulator pixels illuminated by the R1 spectral band are modulatedaccording to image data for the left-eye image and the spatial lightmodulator pixels illuminated by the R2 spectral band are modulatedaccording to image data for the right-eye image.

In the alternate configuration of FIG. 4B, separate uniformizing optics44 and beam scanners 50 are utilized in the light beams from each of thelight sources 42 a and 42 b to provide two scanned light beams. A beamcombiner 46 then combines the scanned light beams to form a combinedscanned light beam, which is directed onto the spatial light modulator60 using illumination optics 90. In this case the beam scanning optics92 includes both beam scanners 50.

The schematic diagram of FIG. 5 shows a stereoscopic digital projectionsystem 100 that has three color channels (i.e., red color channel 40 r,a green color channel 40 g, and a blue color channel 40 b). The redcolor channel 40 r includes spectrally-adjacent red spectral bands R1and R2; the green color channel 40 g includes spectrally-adjacent greenspectral bands G1 and G2; and the blue color channel 40 b includesspectrally-adjacent blue spectral bands B1 and B2. Projection optics 70deliver the imaging light from the three spatial light modulators 60 toa display surface 72. The viewer observes display surface 72 throughfilter glasses 74 having left-eye filter 76L for the left eye andright-eye filter 76R for the right eye. The left-eye filter 76Lselectively transmits the imaging light for the left-eye image (i.e.,light in the R1, G1 and B1 spectral bands), while blocking (by absorbingor reflecting) the imaging light for the right-eye image (i.e., light inthe R2, G2 and B2 spectral bands). Similarly, right-eye filter 76Rselectively transmits the imaging light for the right-eye image (i.e.,light in the R2, G2 and B2 spectral bands), while blocking the imaginglight for the left-eye image (i.e., light in the R1, G1 and B1 spectralbands).

A controller system 80 synchronously modulates the pixels of eachspatial light modulator 60 according to image data for the stereoscopicimage. The controller system 80 is also coupled to the beam scanners 50so that it knows which spatial light modulator pixels are illuminated bythe different spectrally-adjacent bands at any given time. The spatiallight modulator pixels that are illuminated by the first spectral bandare modulated according to image data for the left-eye image and thespatial light modulator pixels that are illuminated by the secondspectral band are modulated according to image data for the right-eyeimage. Since the first and second spectral bands are continuouslyscrolling, the subsets of the spatial modulator pixels that aremodulated with the image data for left-eye and right-eye images arecontinuously changing as well.

Projection optics 70 may combine the light beams from the three colorchannels (e.g., using beam combining optics) and project the combinedbeam through a single projection lens. Alternately, the projectionoptics 70 may use three separate projection lenses to project each ofthe color channels separately onto the display surface 72 in an alignedfashion.

As noted earlier with reference to FIGS. 4A and 4B, the beam scanningoptics 92 including one or more beam scanners 50 can be configured toprovide band of light scrolling using a number of differentarrangements, and can be positioned at any suitable point along theillumination path. Consistent with one embodiment of the presentinvention, FIG. 6A shows a schematic diagram of a beam scanner 50 whichincludes a single scanning element, namely a rotating prism 52. In thisconfiguration, a rotating prism 52 can be provided for each of thespectrally-adjacent spectral bands in each of the component color bands.Rotation of the prism 52 redirects the light beam, shown here for the R1spectral band, by refraction, so that the light beam position iscyclically scrolled across spatial light modulator 60. The FIG. 6Aarrangement is used, for example, in the color channel embodiment shownin FIG. 4B.

In the top diagram of FIG. 6A, the prism 52 is positioned so that theincident beam is normally incident on a face of the prism. In this casethe light beam passes through the prism 52 in an undeflected fashion. Inthe middle diagram, the prism 52 has been rotated around axis O so thatthe light beam is incident at an oblique angle onto the face of theprism. In this case, the beam is refracted downward so that itintersects the spatial light modulator at a lower position. In the lowerdiagram, the prism 52 has been rotated so that the incident beam nowstrikes a different facet of the prism 42. In this case, the beam isrefracted upward so that it intersects the spatial light modulator 60 ata higher position. It should be noted that the incident beam willgenerally have a substantial spatial (and angular) extent so that atsome prism orientations some of the light rays in the incident beam maystrike different faces of the prism. In this way, some of the light rayswill be deflected upwards, while others may be deflected downwards. Thisprovides for the band of light to be split between the upper and lowerportions of the image frame as shown in image frame 38 e of FIG. 3B.

FIG. 6B is a schematic diagram that shows an alternate embodiment forbeam scanner 50, in which a rotating prism 52 simultaneously scans thebands of light for both of the spectrally-adjacent spectral bands in asingle color channel (in this example spectral bands R1 and R2). Thisconfiguration is appropriate for use in the example embodiment of FIG.4A. In this case, light beams for both of the R1 and R2 spectral bandsare incident on the prism 52. As the prism 52 rotates, both of the lightbeams are simultaneously redirected by refraction.

FIG. 6C is a schematic diagram that shows another alternate embodimentfor beam scanner 50, in which a rotating prism 52 simultaneously scansthe bands of light for both of the spectrally-adjacent spectral bands ina single color channel (in this example spectral bands R1 and R2). Inthis case, the beams of light incident on the rotating prism come fromtwo different angular directions. Uniformizing optics 44 are used touniformize each of the spectrally-adjacent light beams. In this example,the uniformizing optics 44 include integrating bars 58. The illuminationoptics 90 are split into a first stage 94 and a second stage 96, eachincluding a plurality of lenses 48. In this configuration, the lenses 48in the first stage 94 are arranged to provide telecentricity between theoutput face of the integrating bars 58 and the prism 52. Similarly, thelenses 48 in the second stage 96 are arranged to provide telecentricitybetween the prism 52 and the spatial light modulator 60. A dichroiccombiner 82, including one or more dichroic surfaces 84, is used todirect the scanned light beams onto parallel optical axes forilluminating the spatial light modulator 60.

The multi-angle geometry of FIG. 6C is similar to that taught by Connerin U.S. Pat. No. 7,147,332, entitled “Projection system with scrollingcolor illumination.” Connor teaches a projection system having ascrolling prism assembly to simultaneously illuminate different portionsof a spatial light modulator with different color bands. White light isdivided into different color bands that propagate through the scrollingprism in different directions. The scrolled color bands are reflectivelycombined so that the different color bands pass out of the scrollingprism assembly parallel. However, Conner does not teach scrollingspectrally-adjacent spectral bands from independent light sources toprovide for stereoscopic projection.

A rotating prism or other refractive element is one type of device thatcan be used for the beam scanner 50. The term “prism” or “prism element”is used herein as it is understood in optics, to refer to a transparentoptical element that is generally in the form of an n-sided polyhedronwith flat surfaces upon which light is incident and that is formed froma transparent, solid material that refracts light. It is understoodthat, in terms of shape and surface outline, the optical understandingof what constitutes a prism is less restrictive than the formalgeometric definition of a prism and encompasses that more formaldefinition. While FIGS. 6A-6C depict a rectangular prism with a squarecross-section, in many instances it is desired to have more than fourfacets in order to provide improved scanning results. For example, ahexagonal prism, or an octagonal prism can be used in variousembodiments.

Alternate types of components that can be utilized for beam scanner 50include rotating mirrors or other reflective components, devices thattranslate across the beam path and provide variable light refraction,reciprocating elements, such as a galvanometer-driven mirror, orpivoting prisms, mirrors, or lenses.

When multiple beam scanners 50 are utilized, it is critical tosynchronize the rotation of all of the beam scanners 50, andsubsequently the image data associated with the different spectralbands. One method, not depicted, is to configure the optical arrangementsuch that a single motor is used to control the moving optical elementsfor at least two of the beam scanners 50. For example a single axle canbe used to drive multiple prisms 52 using a single motor. In someembodiments, a single rotating prism 52 can be used to scan multiplespectral bands by directing light beams through the prism 52 frommultiple directions, or by directing light beam through differentportions of the prism 52 (as shown in FIG. 6B).

As shown in the examples of FIGS. 4A, 4B, and 5, beam paths for thespectrally-adjacent spectral bands can be aligned with each other toilluminate spatial light modulator 60 using the beam combiner 46. Thebeam combiner 46 can be a dichroic beam combiner, or can use any othertype of beam combining optics known in the art.

The uniformizing optics 44 condition the light beams from the lightsources 42 a and 42 b to provide substantially uniform beams of lightfor scanning In the context of the present disclosure, the term“substantially uniform” means that the intensity of the beam of lightincident on the spatial light modulator 20 appears to be visuallyuniform to an observer. In practice, the intensity of the uniformizedlight beams should be constant to within about 30%, with most of thevariation occurring being a lower light level toward the edges of theuniformized light beams. Any type of uniformizing optics 44 known in theart can be used, including integrating bars or lenslet arrays.

FIG. 7A shows an example of uniformizing optics 44 that can be used forthe embodiment of FIG. 4A. The uniformizing optics 44 use a pair oflenslet arrays 54 to uniformized the light beams. One of thespatially-adjacent light beams (e.g., for the R1 spectral band) ispassed through the top half of the lenslet arrays 54, while the otherspatially-adjacent light beam (e.g., for the R2 spectral band) passesthrough the bottom half of the lenslet arrays 54. An opaque block 56 isprovided between the light beams for the spectrally-adjacent spectralbands, to help prevent crosstalk. In this manner a single lenslet arraystructure may be utilized per color band thereby reducing costs.

FIG. 7B shows another example of uniformizing optics 44 that can be usedfor the embodiment of FIG. 4A. In this case, the uniformizing optics 44use a pair of integrating bars 58 to uniformized the light beams. One ofthe spatially-adjacent light beams (e.g., for the R1 spectral band) ispassed through the upper integrating bar 58, while the otherspatially-adjacent light beam (e.g., for the R2 spectral band) passesthrough the lower integrating bar 58.

As mentioned earlier, in a preferred embodiment, the output face(s) ofthe uniformizing optics 44 are imaged onto the spatial light modulator60 using the illumination optics 90, where the imaging light passesthrough the beam scanning optics 92. It will be obvious to one skilledin the art that many different configurations for the illuminationoptics 90 can be used to provide this feature. FIG. 8 shows oneembodiment where the illumination optics 90 are divided into first stage94 and second stage 96, each including two lenses 48. The lenses 48 inthe first stage 94 form an image of the output faces of integrating bars58 at an intermediate image plane 98 corresponding to the position ofthe prism 52, which is a component of the beam scanner 50. The secondstage 96 forms an image of the intermediate image plane 98 onto thespatial light modulator 60, thereby providing substantially-uniformbands of light 36 a and 36 b. The bands of light are scanned across thespatial light modulator as the prism 52 is rotated. The lenses 48 can beused to adjust the magnification of the intermediate image according tothe size of the prism 52, and to adjust the magnification of the scannedbands of light according to the size of the spatial light modulator 60.

The controller system 80 (FIG. 5) synchronously modulates the pixels ofeach spatial light modulator 60 according to image data for thestereoscopic image. Logic in the controller system 80 coordinates theimage data for the left- and right-eye image content with thecorresponding positions of each band of light 36 a and 36 b. Thecontroller system 80 may be a computer or dedicated processor ormicroprocessor associated with the projector system, for example, or maybe implemented in hardware.

Embodiments of the present invention are well suited to usingsolid-state light sources such as lasers, light-emitting diodes (LEDs),and other narrow-band light sources, wherein narrow band light sourcesare defined as those having a spectral bandwidth of no more than about15 nm FWHM (full width half maximum), and preferably no more than 10 nm.Other types of light sources that could be used include quantum dotlight sources or organic light emitting diode (OLED) light sources. Instill other embodiments, one or more white light sources could be used,along with corresponding filters for obtaining the desired spectralcontent for each color channel. Methods for splitting polychromatic orwhite light into light of individual color spectra are well known tothose skilled in the image projection arts and can employ standarddevices such as X-cubes and Phillips prisms, for example, withwell-established techniques for light conditioning and delivery.

The use of lasers provides a significant advantage in reducing thebandwidth of the spectrally-adjacent spectral bands, thereby allowingmore separation between the adjacent bands and increased color gamut.This is desirable in that the filters on each eye are inevitablysensitive to angle whereby the wavelength of the filter edge transitionsshift due to non-normal incidence. This angular sensitivity is acommonly known problem in all optical filter designs. Therefore using areduced bandwidth emission helps to solve this problem enabling thiscommon shift to occur without substantially impacting crosstalk. Manylasers have bandwidths on the order of 1 nm. While this may seem ideal,there are other factors, such as speckle reduction, which benefit frombroader spectral bands. (Speckle is produced by the interference ofcoherent light from defects on optical components.) While speckle canoccur using any type of light source, it is most pronounced with narrowband light sources such as LEDs, and even more so with Lasers. A moredesirable bandwidth would fall between 5-10 nm as a compromise toprovide adequate spectral separation while reducing the sensitivity tospeckle. A spectral separation of between 15-20 nm is generallysufficient to mitigate the filter angular sensitivity issues.

The schematic diagram of FIG. 9 shows a stereoscopic digital projectionsystem 100 using a common optical path for projection optics 70. Thestereoscopic digital projection system includes a red color channel 40r, a green color channel 40 g and a blue color channel 40 b. Each colorchannel includes one or more arrays of light sources (e.g., laser arraysources) for each of a pair of spectrally-adjacent spectral bands. Lightsources 42 a emit light beams in the left-eye spectral bands (R2, G2 andB2), and light sources 42 b emit light in the spectrally-adjacentright-eye spectral bands (R1, G1 and B1). Light-redirecting prisms 30are used in each color channel to redirect the light beams from thelight sources 42 a and 42 b into a common direction to form a combinedlight beam including spatially-adjacent light beams for the right-eyeand left-eye spectral bands (e.g., the R1 and R2 spectral bands). Thelight beams from the right-eye spectral band (e.g., the R1 spectral)will be grouped on one side of the combined light beam, and the lightbeams from the left-eye spectral band (e.g., the R2 spectral) will begrouped on the other side of the combined light beam. One type oflight-redirecting prism 30 that can be used for this purpose isdescribed in the aforementioned, commonly-assigned, co-pending U.S.Patent Application Publication 2009/0153752 entitled “Projector usingindependent multiple wavelength light sources” by Silverstein, which isincorporated herein by reference.

The combined light beam for each component color channel is directedthrough uniformizing optics 44, beam scanning optics 92 and illuminationoptics 90, and is reflected from dichroic surface 68 to provide scannedfirst and second bands of light 36 a and 36 b onto the correspondingspatial light modulators 60. A controller system 80 (FIG. 5)synchronously modulates the spatial light modulator pixels according toimage data for the stereoscopic image, wherein the spatial lightmodulator pixels illuminated by the first band of light (e.g., R1) aremodulated according to image data for the left-eye image and the spatiallight modulator pixels illuminated by the second band of light (e.g.,R2) are modulated according to image data for the right-eye image.

The modulated imaging light beams provided by the spatial lightmodulators 60 are transmitted through the dichroic surfaces 68 and arecombined onto a common optical axis using a dichroic combiner 82 havingmultiple dichroic surfaces 84. The combined light beam is projected ontoa display surface (not shown) using the projection optics 70 for viewingby observers wearing filter glasses 74 (FIG. 5).

The embodiment illustrated in FIG. 9 uses three spatial light modulators60, one for each component color channel (i.e., red, green and blue).Each spatial light modulator 60 is illuminated with scrolling bands oflight having spectrally-adjacent spectral bands within a particularcomponent color channel. The spatial light modulators tend to be one ofthe more expensive and complex components of the stereoscopic digitalprojection system 100.

FIG. 10 illustrates a schematic diagram for an alternate embodiment of astereoscopic digital projection system 110 that utilizes only twospatial light modulators 60L and 60R, one associated with a left-eyeimage forming system 41L and one associated with a right-eye imageforming system 41R. The left-eye image forming system 41L includes threeleft-eye light sources 43L, one for each component color spectrum (R1,G1 and B1). Similarly, the right-eye image forming system 41R includesthree right-eye light sources 43R, one for each component color spectrum(R2, G2 and B2). The right-eye light sources 43R are spectrally-adjacentto the corresponding left-eye light sources 43L.

Each of the image forming systems include uniformizing optics 44, beamscanning optics 92, illumination optics 90 and a dichroic surface 68 todirect the scanned beams of light onto spatial light modulators 60L and60R. In this case, the left-eye image forming system 41L provides threescanned bands of light 34 r, 34 g and 34 b, corresponding to the red,green and blue spectral bands (R1, G1 and B1), respectively. Likewise,the right-eye image forming system 41R provides three scanned bands oflight 35 r, 35 g and 35 b, corresponding to the red, green and bluespectral bands (R2, G2 and B2), respectively.

A controller system (not shown) synchronously modulates the pixels ofthe spatial light modulator 60L in the left-eye image forming system 41Laccording to image data for the left-eye image, wherein the pixelsilluminated by the each band of light (R1, G1 and B1) are modulatedaccording to the image data for the corresponding color channel of theleft-eye image. Likewise, the controller system synchronously modulatesthe pixels of the spatial light modulator 60R in the right-eye imageforming system 41R according to image data for the right-eye image,wherein the pixels illuminated by the each band of light (R2, G2 and B2)are modulated according to the image data for the corresponding colorchannel of the left-eye image.

A dichroic combiner 82 including a dichroic surface 84 is used tocombine the imaging light from the left-eye image forming system 41L andthe right-eye image forming system 41R onto a common optical axis forprojection onto a display surface using projection optics 70. Thedichroic surface 84 is preferably a spectral comb filter having a seriesof notches that transmits the spectral bands (R2, G2 and B2)corresponding to the imaging light for the right-eye light sources 43Rwhile reflecting the spectral bands (R1, G1 and B1) corresponding to theimaging light for the left-eye light sources 43L. Spectral comb filterscan be fabricated using any technique known in the art, such asmulti-layer thin-film dichroic filter coating methods and co-extrudedstretched polymer film structure fabrication methods. Another type ofdichroic filter that can be used to provide a spectral comb filter foruse as dichroic surface 84 is a rugate filter design. Rugate filters areinterference filters that have deep, narrow rejection bands while alsoproviding high, flat transmission for the rest of the spectrum. Rugatefilters are fabricated using a manufacturing process that yields acontinuously varying index of refraction throughout an optical filmlayer. Rugate filters feature low ripple and no harmonic reflectionscompared to standard notch filters, which are made with discrete layersof materials with different indices of refraction.

By way of example, and not by way of limitation, Tables 1 and 2 listexample spectrally-adjacent spectral bands according to embodiments ofthe present invention.

TABLE 1 Exemplary interleaved spectrally-adjacent spectral bandsRight-Eye Image Left-Eye Image Component Color Spectral Bands SpectralBands Red 625-640 nm 655-670 nm Green 505-520 nm 535-550 nm Blue 442-456nm 470-484 nm

TABLE 2 Exemplary non-interleaved spectrally-adjacent spectral bandsRight-Eye Image Left-Eye Image Component Color Spectral Bands SpectralBands Red 625-640 nm 655-670 nm Green 535-550 nm 505-520 nm Blue 442-456nm 470-484 nm

FIG. 11A shows spectral bands R1, G1, and B1 for the right eye andspectral bands R2, G2, and B2 for the left eye for each component coloraccording to the Table 1 arrangement. Each of the spectral bands has acorresponding central wavelength (λ_(R1), λ_(G1), λ_(B1), λ_(R2),λ_(G2), λ_(B2)) and a corresponding bandwidth (W_(R1), W_(G1), W_(B1),W_(R2), W_(G2), W_(B2)). For the FIG. 11A arrangement, the spectralbands observe an interleaved ordering according to the centralwavelengths for the respective spectral bands:λ_(B1)<λ_(B2)<λ_(G1)<λ_(G2)<λ_(R1)<_(R2).

The bandwidths can be characterized using an appropriate measure ofwidth for the spectral bands. Typically, the bandwidths are defined tobe the wavelength separation between the lower edge (i.e., the “cut-onedge”) of the spectral band and the upper edge (i.e., the “cut-offedge”) of the spectral band. In a preferred embodiment, the bandwidthsare full-width half-maximum bandwidths where the lower and upper edgescorrespond to the wavelengths where the spectral power in the spectralband falls to half of its peak level. In other embodiments the lower andupper edges can be determined according to other criteria. For examplethe edges can be defined to be the wavelengths where the spectral powerfalls to a specified level other than half of the peak level (e.g., the10% power level or the 25% power level). Alternatively, the bandwidthcan be characterized using some other measure of the width of thespectral band (e.g., a multiple of the standard deviation of thespectral power distribution for the spectral band).

In the example shown in FIG. 11A, the bandwidth of each spectral band isabout 10-15 nm, while the separation between adjacent spectral bands is15 nm or more. Various embodiments may use light emitters havingdifferent bandwidths, or may have different separations between theadjacent spectral bands. The minimum bandwidth for typical lightemitters that would be used for digital projection systems would beabout 1 nm, corresponding to the bandwidth of a single laser.

The central wavelengths of the spectral bands can be characterized usingany appropriate measure of the central tendency for the spectral bands.For example, in various embodiments, the central wavelengths can be peakwavelengths of the spectral bands, centroid wavelengths of the spectralbands, or the average of the lower and upper edge wavelengths.

FIG. 11B shows spectral bands R1, G1, and B1 for the right eye andspectral bands R2, G2, and B2 for the left eye for each component coloraccording to the Table 2 arrangement. In this case, the spectral bandsobserve a non-interleaved ordering according to the central wavelengthsfor the respective spectral bands where:λ_(B1)<λ_(B2)<λ_(G2)<λ_(G1)<λ_(R1)<λ_(R2). The rearrangement of the G1and G2 spectral bands in the FIG. 11B arrangement relative to theordering in the FIG. 11A arrangement is generally advantageous forsimplifying filter glass coating design and for other purposes, as willsubsequently be described in more detail.

It should be noted that there will generally be slight color gamutdifferences between right- and left-eye imaging paths associated withthe use of the different red, green and blue primaries. As a result,different color processing, including white balance and color correctiontransforms, will generally be needed to account for the spectral bandsassociated with the primary colors used for left-eye and right-eyeimaging paths. White balancing can be performed, for example, byadjusting the brightness of one or more light emitters, by applyingtransforms to individual color channels, by adjusting illuminationtiming or by using filtration to adjust color intensity. Colorcorrection transforms are used to determine control signals for each ofthe color channels to produce a desired color appearance associated witha set of input color values. Color correction transforms will generallyalso include some form of gamut mapping to determine appropriate outputcolors for cases where the input color values are outside of the colorgamut associated with the color primaries used for the left-eye andright-eye imaging paths. Color correction operations can be performed byapplying color correction matrices, or by applying other forms of colortransforms such as three-dimensional look-up tables (3-D LUTs). Methodsfor determining color transforms that are appropriate for a particularset of color primaries are well-known in the art.

Because additional spectral bands are available for wavelength-basedstereoscopic imaging systems, there may be additional color gamutavailable that can be utilized when the system is used fornon-stereoscopic imaging applications. An example of a technique thatcan be used for this purpose is described in commonly assigned U.S.Patent Application Publication No. 2011/0285962 entitled “2D/3DSwitchable Color Display Apparatus with Narrow Band Emitters” byEllinger et al.

The right-eye filter 76R and the left-eye filter 76L in filter glasses74 (FIG. 5) have spectral transmittance characteristics that aredesigned to transmit the spectral bands associated with thecorresponding left-eye or right-eye image and block the spectral bandsassociated with the other eye. FIG. 12A illustrates an example of aright-eye filter transmittance 78R for right-eye filter 76R and aleft-eye filter transmittance 78L for left-eye filter 76L that can beused in accordance with the interleaved spectral band arrangement shownin FIG. 11A. The right-eye filter transmittance 78R transmits most ofthe light in the right-eye spectral bands (R1, G1, B1) while blockingmost of the light in the left-eye spectral bands (R2, G2, B2). Likewise,the left-eye filter transmittance 78L transmits most of the light in theleft-eye spectral bands (R2, G2, B2) while blocking most of the light inthe right-eye spectral bands (R1, G1, B1). In this example, both theright-eye filter transmittance 78R and the left-eye filter transmittance78L is a “comb filter” that includes two contiguous bandpass filtertransmission bands 77B and one contiguous edge filter transmission band77E. A transmission band is considered to be contiguous provided that ithas at least some minimum specified transmission percentage (e.g., 50%)over all wavelengths within the transmission band.

The right-eye filter 76R and the left-eye filter 76L should generally bedesigned to transmit at least 50% of the light from the correspondingeye spectral bands in order to avoid causing a significant loss in imagebrightness. Preferably, this value should be 80% or higher. To preventobjectionable cross-talk, the right-eye filter 76R and the left-eyefilter 76L should generally be designed to transmit less than 5% of thelight from the opposite eye spectral bands. Preferably, this valueshould be less than 2% to ensure that the crosstalk is substantiallyimperceptible.

FIG. 12B illustrates an example of a right-eye filter transmittance 79Rfor right-eye filter 76R and a left-eye filter transmittance 79L forleft-eye filter 76L that can be used in accordance with thenon-interleaved spectral band arrangement shown in FIG. 11B. Incomparison to the arrangement shown in FIG. 12B, it can be seen that thefilters in the arrangement of FIG. 12A have the advantage that theyrequire fewer edge transitions. In particular, both the right-eye filtertransmittance 78R and the left-eye filter transmittance 78L use only asingle bandpass filter transmission band 77B, together with a singleedge filter transmission band 77E. This is made possible by the factthat due to the reordering of the spectral bands there is no interveningleft-eye spectral band between the right-eye green spectral band G1 andthe right-eye red spectral band R1. Likewise, there is no interveningright-eye spectral band between the left-eye blue spectral band B2 andthe left-eye green spectral band G2. Each of the filters in thearrangement of FIG. 12B require only three edge transitions (from lowtransmittance to high transmittance or from high transmittance to lowtransmittance), whereas the filters in the arrangement of FIG. 12A eachrequire five edge transitions. In general, the complexity of a filterdesign increases with the number of edge transitions, and with thesharpness of the edge transitions that are required. The fabrication offilters with fewer bandpass filter transmission bands (and thereforefewer edge transitions) is therefore significantly less complex,requiring fewer filter layers, and as a result is less expensive. Thisis an important advantage since the filter glasses 74 must bemanufactured in large quantities for use by each viewer in the audiencewho is viewing the projected stereoscopic image. Another advantage ofthe arrangement of FIG. 12B is that there are fewer opportunities forgenerating crosstalk since there are fewer edge transitions where anopposing eye spectral band can leak into a transmission band.

The left-eye filter 76L and the right-eye filter 76R in filter glasses74 (FIG. 5) can be made using any fabrication technique known in theart. In some embodiments, one or both of the left-eye filter 76L and theright-eye filter 76R are dichroic filters that includes an opticalsurface having a multi-layer thin-film coating. The multi-layerthin-film coating can be designed to provide appropriate filtertransmittances, such as the right-eye filter transmittance 78R and theleft-eye filter transmittance 78L of FIG. 12A and the right-eye filtertransmittance 79R and the left-eye filter transmittance 79L of FIG. 12B.Techniques for designing and fabricating multi-layer thin-film coatingshaving specified spectral transmittance characteristics are well knownin the art.

In other embodiments, one or both of the left-eye filter 76L and theright-eye filter 76R are multi-layer dichroic filters that arefabricated using a co-extruded stretched polymer film structure. Onemethod for fabricating such structures is described in U.S. Pat. No.6,967,778 to Wheatley et al., entitled “Optical film with sharpenedbandedge,” which is incorporated herein by reference. According to thismethod, a coextrusion device receives streams of diverse thermoplasticpolymeric materials from a source such as a heat plastifying extruder.The extruder extrudes a multi-layer structure of the polymericmaterials. A mechanical manipulating section is used to stretch themulti-layer structure to achieve the desired optical thicknesses.

Crosstalk is an undesirable artifact that can occur in stereoscopicimaging systems where the image content intended for one of theobserver's eyes is contaminated with the image content intended for theother eye. This can create the appearance of perceptible “ghost images”where the viewer sees faint images of objects in the scene that arespatially offset from the main images. To avoid objectionable crosstalkit is important that the amount of light from the left-eye lightemitters 12L that is transmitted by the right-eye filter 76R is a smallfraction of the amount of light from the right-eye light emitters 12Rthat is transmitted by the right-eye filter 76R. Likewise, the amount oflight from the right-eye light emitters 12R that is transmitted by theleft-eye filter 76L should be a small fraction of the amount of lightfrom the left-eye light emitters 12L that is transmitted by the left-eyefilter 76L.

FIG. 13 illustrates the origin of crosstalk in a wavelength-basedstereoscopic imaging system. The figure shows a close up of thewavelength range that includes the right-eye red spectral band R1 andthe left-eye red spectral band R2. A left-eye filter transmittance 79Lis shown that transmits the majority of the light in the left-eye redspectral band R2 while blocking the majority of the light in theright-eye red spectral band R1. However, it can be seen that there is asmall overlap region 75 where a small amount of the light from theright-eye red spectral band R1 is transmitted by the left-eye filtertransmittance 79L. This transmitted right-eye light will reach theobserver's left eye, producing crosstalk and resulting in a faint ghostimage.

Various metrics can be used to characterize the amount of crosstalk. Onesuch metric is given by the following equation:

$\begin{matrix}{C_{Rarrow L} = {\frac{\int{{P_{R}(\lambda)}{T_{L}(\lambda)}{\lambda}}}{\int{{P_{L}(\lambda)}{T_{L}(\lambda)}{\lambda}}} \times 100}} & ( {1A} ) \\{C_{Larrow R} = {\frac{\int{{P_{L}(\lambda)}{T_{R}(\lambda)}{\lambda}}}{\int{{P_{R}(\lambda)}{T_{R}(\lambda)}{\lambda}}} \times 100}} & ( {1B} )\end{matrix}$

where C_(R→L)is the amount of crosstalk from the right-eye image thatcontaminates the left-eye image, C_(L→R)is the amount of crosstalk fromthe left-eye image that contaminates the right-eye image, P_(L)(λ) andP_(R)(λ) are the spectral power distributions for the light from theleft-eye light emitters 12L and the right-eye light emitters 12R,respectively, T_(L)(λ) and T_(R)(λ) are the spectral transmittances forthe left-eye filter 76L and the right-eye filter 76R, respectively, andλ is the wavelength. It can be seen that the metrics given by Eqs. (1A)and (1B) compute the percentages of the undesired light that is passedby the filters relative to the amount of desired light that is passed bythe filters. Generally, the amount of crosstalk should be less than 5%under all viewing conditions to avoid objectionable artifacts, andpreferably it should be less than 2% to ensure that the crosstalk issubstantially imperceptible.

A number of factors influence the level of crosstalk that occurs in thestereoscopic digital projection system 110 (FIG. 2). These factorsinclude the amount of wavelength separation between the left-eyespectral bands and the right-eye spectral bands, the sharpness of theedge transitions for the light-emitter spectral bands, the sharpness ofthe edge transitions for the filter transmission bands, and thealignment between the light-emitter spectral bands and the filtertransmission bands. Since the locations of the edge transitions for thefilter transmission bands is sometimes a function of the incidence angle(e.g., for dichroic filters), the amount of crosstalk may be a functionof viewing angle.

The wavelength separation between the left-eye spectral bands and theright-eye spectral bands is a particularly important factor that must beconsidered during the design of a digital projection system in order toavoid crosstalk. The wavelength separation can be defined to be thewavelength interval between the upper edge (i.e., the “cut-off edge”) ofthe lower spectral band to the lower edge (i.e., the “cut-on edge”) ofthe higher spectral band. This distance is characteristically measuredfrom the half-maximum point on each band edge. For example, FIG. 13shows the wavelength separation S between the right-eye red spectralband R1 and the left-eye red spectral band R2. The amount of wavelengthseparation that is necessary to eliminate objectionable crosstalk willdepend on the sharpness of the edge transitions in the filtertransmittance, as well as other effects such as variability of the edgetransition location with incidence angle.

The variation of the locations of the edge transitions with angle ofincidence for a set of commercially available filters intended for usewith wavelength-based stereoscopic imaging systems is illustrated inFIG. 14. Graph 130 shows a pair of measured spectral transmittancecurves for a right-eye filter for normally incident light as well aslight incident at a 20° angle of incidence. It can be seen that the edgetransitions shift about 5-10 nm toward the short wavelength direction.These wavelength shifts occur as a result of the longer path length thatthe light takes through the dichroic filter stack. Since the shiftsoccur towards the short wavelength direction, they are sometimes called“blue shifts.” Graph 135 shows an analogous pair of spectraltransmittance curves for a left-eye filter, which exhibit similar shiftsin the edge transitions.

Because of the variability in the locations of the edge transitions, itis generally desirable that the wavelength separation between theleft-eye spectral bands and the right-eye spectral bands be large enoughto accommodate the range of edge transition positions associated withthe range of expected viewing angles without inducing objectionablecrosstalk artifacts. U.S. Patent Application Publication No.2010/0060857, entitled “System for 3D Image Projection Systems andViewing,” to Richards et al. notes this problem and recommends sizing“guard bands” or notches between the respective spectral bands for eacheye, such as between the green color channel spectral bands G1 and G2,for example.

In a preferred embodiment, the light emitters are narrow-band lightsources, such as solid-state lasers, having bandwidths that are no morethan about 15 nm. Accordingly, if the central wavelengths of eachspectral band for a particular color are chosen to be at least 25 nmapart, this will provide wavelength separations between the bands of atleast 10-15 nm, which is sufficient to provide substantial protectionagainst crosstalk given properly designed filters. For this and otherreasons, the use of narrow-band solid state light sources is advantagedover conventional approaches that use filtered white light sources,wherein the bandwidths of the spectral bands typically exceed 40 nm forindividual primary colors. (The larger bandwidth is necessary inconventional filtered white light sources as further narrowing of thespectrum reduces the system optical efficiency.)

The left-eye filter 76L and the right-eye filter 76R can be made usingany spectral filter technology known in the art. One type of spectralfilters of particular interest for wavelength-based stereoscopic imagingsystems are dichroic filters made using thin-film dichroic filterstacks. Dichroic filters are fabricated by coating a plurality oftransparent thin film layers having markedly different refractiveindices on a substrate. The thin film layers can be deposited in variousforms and using various methods, including vacuum coating andion-deposition, for example. The material is deposited in alternatinglayers having thicknesses on the order of one-quarter wavelength of theincident light in the range for which the coating is designed. Materialsused for the coating layers can include dielectrics, metals, metallicand non-metallic oxides, transparent polymeric materials, orcombinations thereof. In an alternate embodiment, one or more of thedichroic filter stack layers is deposited as a solution ofnanoparticles. Where polymer materials are used, one or more of thefilter stack layers can be formed from extruded materials.

The thicknesses and indices of refraction of the thin film layers in thedichroic filter stack can be adjusted to control the spectraltransmittance characteristics. One important advantage of using filtersmade using dichroic filter stacks is that given enough layers, the shapeof the spectral transmittance curves can be accurately controlled, andvery sharp edge transitions can be achieved. This enables filters to beprovided that selectively transmit one set of spectral bands whileblocking the other set.

However, one characteristic of dichroic filters that can bedisadvantageous for stereoscopic imaging application is that the lightthat is not transmitted through the filter is reflected back off thefilter. The undesirable effects of this effect is illustrated in FIG.15A. Imaging light from display surface 72 is directed toward anobserver wearing filter glasses 74 (not shown in FIG. 15A) that includeright-eye filter 76R disposed in front of the right eye 194 of theobserver. The right-eye filter 76R in this case includes a dichroicfilter stack 86 on a front surface 66F of a transparent substrate 88,such as a glass or plastic substrate. (The front surface 66F faces thedisplay surface 72, while the opposite rear surface 66R faces theobserver.)

The incident light includes right-eye incident light 196R comprisingright-eye image data and left-eye incident light 196L comprisingleft-eye image data. The right-eye incident light 196R is substantiallytransmitted through the right-eye filter 76R as right-eye transmittedlight 198R and will be incident on the observer's right eye 194 toenable the observer to view the right-eye image. The left-eye incidentlight 196L is substantially reflected back into the viewing environmentas left-eye reflected light 197L. This reflected light can be scatteredaround in the viewing environment and can contaminate the viewed imageas “flare” light that would be transmitted through the left-eye filter76L (FIG. 5) into the observer's left eye. The problem of flare light isexacerbated as the audience size increases. The reflected light fromeach pair of filter glasses 74 can be inadvertently directed back towardthe display screen or to other objects or structures in the viewingarea, increasing the amount of visual noise and reducing image contrast.

Some of the left-eye flare light from a direction behind the observermay be incident on rear surface 66R of the right-eye filter 76R. Thislight is shown as left-eye incident light 186L, which will besubstantially reflected from the dichroic filter stack and will bedirected back into the right eye 194 as left-eye reflected light 187L.The origin of this light may be direct reflections off the filterglasses 74 worn by other viewers that are seated behind the observer, ormay be light that may have been reflected off of other surfaces.

The left-eye filter 76L, which is not shown in FIG. 15A, has a similarstructure and complementary behavior, substantially transmitting theintended image-bearing light emitted for the left-eye image whilesubstantially blocking the unwanted light for the right-eye image.

FIG. 15B shows an arrangement similar to that shown in FIG. 15A wherethe dichroic filter stack is on the rear surface 66R of the substrate88. The overall behavior of the right-eye filter 76R is identical tothat of FIG. 15A, although this configuration has the advantage that thedichroic filter stack 86 may be less likely to be damaged by scratchingit since it is less exposed.

To further illustrate the problem of unwanted reflected light, FIG. 16Ashows a typical right-eye dichroic filter transmittance 170R that can beused with a wavelength-based stereoscopic projection system that usesright-eye light emitters having right-eye spectral bands R1, G1 and B1and left-eye light emitters having left-eye spectral bands R2, G2 andB2. It can be seen that dichroic filter transmittance 170R is arrangedto transmit most of the light in the right-eye spectral bands R1, G1,B1, while blocking most of the light in the left-eye spectral bands R2,G2, B2.

FIG. 16B is a graph showing the light that is transmitted through theright-eye filter 76R according to the right-eye dichroic filtertransmittance 170R as a function of wavelength. In this example, thetransmitted right-eye light 175R includes more than 90% of the incidentlight in the right-eye bands, and the transmitted left-eye light 175Lincludes about 3% of the light in the left-eye spectral bands. Asdiscussed earlier, the transmitted left-eye light 175L will be a sourceof crosstalk in the viewed stereoscopic image.

For dichroic filters, the dichroic filter reflectance R_(D)(λ) will beapproximately equal to:

R _(D)(λ)≈(1−T _(D)(λ))   (2)

where T_(D)(λ) is the dichroic filter transmittance. FIG. 16C shows aright-eye dichroic filter reflectance 171R corresponding to theright-eye dichroic filter transmittance of FIG. 16A. It can be seen thatthe right-eye dichroic filter reflectance 171R reflects the majority ofthe light in the left-eye spectral bands R2, G2, B2.

FIG. 16D is a graph showing the light that is reflected from theright-eye filter 76R according to the right-eye dichroic filtertransmittance 170R as a function of wavelength. In this example, thereflected right-eye light 176R includes less than 10% of the incidentlight in the right-eye bands, and the reflected left-eye light 176Lincludes about 97% of the light in the left-eye spectral bands. Asdiscussed earlier, this reflected light can be a source of objectionableflare in the viewing environment.

In some embodiments, the problem of unwanted reflected light ismitigated using a hybrid filter design as shown in FIG. 17A. With thisapproach, the right-eye filter 76R includes both a dichroic filter stack86, as well at least one wavelength-variable absorptive filter layer 87.Absorptive filters absorb a fraction of the light at a particularwavelength, while transmitting the remainder of the light. (Some smallfraction of the light may also be reflected.) It is generally notpossible to produce absorptive filters having spectral transmittancecharacteristics with sharp edge transitions at arbitrary wavelengths ascan be done with dichroic filter designs. Therefore, absorptive filtersare typically not suitable to provide the high degree of colorseparation required for wavelength-based stereoscopic imaging system.However, the combination of absorptive filter layers with dichroicfilter layers has been found to provide significant performanceadvantages relative to the use of pure dichroic filters.

In accordance with embodiments of the present invention, the dichroicfilter stack is designed to transmit 60% or more of the light from theright-eye light emitters and reflect 60% or more of the light from theleft-eye light emitters. Preferably, the dichroic filter stack shouldtransmit at least 90% or more of the light from the right-eye lightemitters and reflect at least 90% of the light from the left-eye lightemitters.

Likewise, the absorptive filter layers 87 are designed to transmit alarger percentage of the light in the right-eye spectral bands that thelight in the left-eye spectral bands. Preferably, the absorptive filterlayers 87 should transmit a large majority of the light in the right-eyespectral bands, while absorbing a large majority of the light in theright-eye spectral bands.

Taken together, the hybrid right-eye filter is adapted to transmit 50%or more of the light from the right-eye light emitters, while blockingmost of the light in the left-eye light emitters so that the amount oftransmitted light from the left-eye light emitters is less than 5% ofthe transmitted light from the right-eye light emitters. The absorptioncharacteristics of the absorptive filter layers 87 are such that theamount of left-eye incident light 196L reflected from the right-eyefilter 76R is substantially reduced relative to configurations that useonly a dichroic filter stack 86 (e.g., the configurations shown in FIGS.15A-15B). In a preferred embodiment, the right-eye filter 76R shouldabsorb a majority of the left-eye incident light 196L such that lessthan 50% of the left-eye incident light 196L is reflected. Ideally, theright-eye filter 76R should absorb a large majority (e.g., more than90%) of the left-eye incident light 196L.

In some embodiments, the absorptive filter layers 87 can be coated ontop of the dichroic filter stack 86. In other embodiments, theabsorptive filter layers 87 can be provided by doping the thin filmlayers or substrate.

Wavelength-variable absorptive materials that are useful for providingabsorptive filter layers 87 include relatively narrow-band absorbingdyes and pigments, such as ABS 647 and ABS 658 available from Exciton ofDayton Ohio; Filtron A Series dye absorbers and Contrast Enhancementnotch absorbers available from Gentex Corp. of Simpson, Pa., or othermolecular chemistries.

Other classes of wavelength-variable absorptive materials that can beused in accordance with the present invention include metamaterials orresonant plasmonic structures. Metamaterials are structurally shapednano-structures that can be tuned to absorb light. An example of such amaterial is described by Padilla in the article entitled “Newmetamaterial proves to be a ‘perfect’ absorber of light” (Science Daily,May 29, 2008). Similarly, plasmonic absorbers have been created by useof typically reflective metals structured at sub-wavelength scales suchthose described by Aydin et al. in the article “Broadbandpolarization-independent resonant light absorption using ultrathinplasmonic super absorbers” (Nature Communications, pp. 1-7, Nov. 1,2011).

Still other absorber structures can be utilized such as photoniccrystals where photonic crystals are utilized to guide light throughmultiple passes through absorption materials. For example, Zhou et al.describe absorption enhancements using photonic crystals in the article“Photonic crystal enhanced light-trapping in thin film solar cells”(Journal of Applied Physics, Vol. 103, paper 093102, 2008).

Still another approach to spectral filtration uses naturally derivednanoparticle absorbers such as colored films created by dipping asubstrate in a solution of viruses or protein molecules. In someembodiments, the virus or protein molecules can be self-assembling. Oneexample of absorbers using nonparticle virus molecules has beendeveloped by Seung-Wak Lee at University of California, Berkeley and isdescribed in an article entitled “No paint needed! Virus patternsproduce dazzling colour” (New Scientist, p. 18, Oct. 29, 2011).

In some embodiments, a plurality of absorptive filter layers 87 can beused. For example, individual absorptive filter layers 87 can beprovided for to selectively absorb light in each of the spectral bandsR2, G2, B2 that comprise the left-eye incident light 196L. Alternately,a single absorptive filter layer 87 can be used to selectively absorblight in portions of a plurality of the spectral bands R2, G2, B2.

In the configuration of FIG. 17A, the dichroic filter stack 86 ispositioned over the front surface 66F of the substrate 88, and theabsorptive filter layer 87 is positioned over the dichroic filter stack86. In order to achieve the stated advantages the absorptive filterlayer 87 must be positioned between the light source (e.g., the displaysurface 72) and the dichroic filter stack 86 so that the unwanted lightis absorbed before it can be reflected by the dichroic filter stack 86.As illustrated in FIG. 17B, the absorptive filter layer 87 and thedichroic filter stack 86 can alternatively be positioned in otherarrangements as long as they maintain the proper relative positions. Inthis example, the dichroic filter stack 86 is positioned over the rearsurface 66R while the absorptive filter layer 87 is positioned over thefront surface 66F.

The arrangements of FIGS, 17A and 17B will be ineffective to prevent thereflection of left-eye incident light 186L that is incident on the rearsurface 66R of the right-eye filter 76R (e.g., after reflecting off offilter glasses worn by other viewers). This light will interact with thedichroic filter stack 86 before it reaches the absorptive filter layer87, and will therefore still be reflected as left-eye reflected light187L.

FIGS. 17C and 17D show arrangements that are analogous to FIGS. 17A and17B, respectively, where a second absorptive filter layer 86 ispositioned over the rear surface 66R. In this way, both the left-eyeincident light 196L and the left-eye incident light 186L will besubstantially absorbed, although at the cost of a slightly lowertransmittance for the right-eye incident light 196R. In suchembodiments, the right-eye filter 76R should preferably absorb amajority of the left-eye incident light 186L such that less than 50% ofthe left-eye incident light 186L is reflected. Ideally, the right-eyefilter 76R should absorb a large majority (e.g., more than 90%) of theleft-eye incident light 186L.

In other embodiments, the layers can be distributed in otherarrangements, or can be combined with additional layers. For example,additional protective layers can be positioned over one or both of thedichroic filter stack 86 or the absorptive filter layer 87 to providescratch resistance or fade resistance. An anti-reflection coating canalso be used to reduce first-surface reflections. In some embodiments,the anti-reflection coating can be formed with a plurality of thin filmlayers, which can optionally be included as part of the dichroic filterstack 86.

FIG. 18 shows an example of a right-eye absorptive filter transmittance172R that can be used for the absorptive filter layer 87 (FIG. 17A), incombination with the right-eye dichroic filter transmittance 170R ofFIG. 16A. If a dichroic filter stack 86 and an absorptive filter layer87 with these spectral properties are used in the hybrid filterarrangement of FIG. 17A or 17B, the combined transmittance of the hybridfilter T_(H)(λ) can be calculated as follows:

T _(H)(λ)≈T _(D)(λ)T _(A)(λ) (3)

where T_(D)(λ) is the dichroic filter transmittance and T_(A)(λ) is theabsorptive filter transmittance. (This assumes that the substratetransmittance is approximately equal to 1.0.) The combined reflectanceof the hybrid filter R_(H)(λ) can be calculated as follows:

R _(H)(λ)=R _(D)(λ)(T _(A)(λ))²=(1−T _(D)(λ))(T _(A)(λ))²   (4)

where R_(D)(λ) is the dichroic filter reflectance, which is equal to1−T_(D)(λ) by Eq. (2). This equation is based on the assumption that thereflected light is transmitted through the absorptive filter layer 87,reflected by the dichroic filter stack 86, and then transmitted throughthe absorptive filter layer 87 a second time. It makes the assumptionthat first surface reflectances can be neglected.

FIG. 19A shows a right-eye hybrid filter transmittance 173R calculatedfrom the spectral transmittances in FIG. 18 using Eq. (3). The right-eyehybrid filter transmittance 173R is superimposed on a set of right-eyespectral bands R1, G1 and B1 and a set of left-eye spectral bands R2, G2and B2. Comparing FIG. 19A to FIG. 16A, it can be seen that theright-eye hybrid filter transmittance 173R is quite similar to theright-eye dichroic filter transmittance 170R.

FIG. 19B is a graph showing the light that is transmitted through theright-eye filter 76R according to the right-eye hybrid filtertransmittance 173R as a function of wavelength. In this example, thetransmitted right-eye light 175R includes about 81% of the incidentlight in the right-eye bands, which is a slight degradation relative tothe dichroic-only configuration that was plotted in FIG. 16B. However,the transmitted left-eye light 175L includes only about 1% of the lightin the left-eye spectral bands. This represents about a 3× reduction inthe amount of cross-talk relative to the dichroic-only configuration.This reduction in cross-talk is an added benefit of the hybrid filterapproach.

FIG. 19C shows a right-eye hybrid filter reflectance 174R calculatedusing Eq. (4). In comparison to FIG. 16C, it can be seen that thereflectivity in the wavelength regions corresponding to the left-eyespectral bands R2, G2, B2 is significantly reduced.

FIG. 19D is a graph showing the light that is reflected from theright-eye filter 76R according to the right-eye hybrid filterreflectance 174R as a function of wavelength. In this example, thereflected right-eye light 176R includes less than 7% of the incidentlight in the right-eye bands, and the reflected left-eye light 176Lincludes about 8% of the light in the left-eye spectral bands. Thisrepresents more than a 12× reduction in the amount of reflected left-eyelight relative to the dichroic-only solution. This will provide asignificant reduction in the amount of flare light that results fromreflections off the filter glasses 74.

It should be noted that absorptive filter layers 87 can be used tosupplement the spectral separation provided by dichroic filter stacks 86to form hybrid filters whether the stereoscopic imaging system usesinterleaved spectral bands (as in the examples discussed relative toFIG. 12A and FIGS. 19A-19D) or non-interleaved spectral bands (such asthe configuration shown in FIG. 12B). A general design principle is thatthe absorptive filter layers 87 used with the filter for a particulareye should absorb more of the spectral bands associated with theopposite eye image and less of the spectral bands associated with theimage-forming light for the particular eye.

As has been noted, reflection of “flare light” that is reflected fromfilter glasses 74 worn by other viewers can reduce the contrast of theprojected image seen by an observer and can add visual noise thatdetracts from the stereoscopic viewing experience. To illustrate this,FIGS. 20A and 20B illustrate a scenario where some incoming light 230from display surface 72 is reflected from filter glasses 74 worn by arear observer 160 and is directed as reflected light 235 onto the rearside of filter glasses 74 worn by a front observer 162. As was discussedrelative to FIGS. 15A and 15B, some of this light can be reflected backinto the eyes of front observer 162. This effect can be more or lesspronounced, depending on whether or not the heads of rear observer 160and front observer 162 are at the same height as shown in FIG. 20A, orat different heights as shown in FIG. 20B. With typical seatingarrangements, the head of front observer 162 is at a lower elevationthan that of the rear observer 160 as shown in the FIG. 20Bconfiguration. In the worst case scenario, the filter glasses 74 usedichroic filters that reflect most or all of the light from the spectralbands that are not transmitted to the eyes of the rear observer 160.When front observer 162 is directly in front and relatively level withrear observer 160, those functionally identical filter glasses 74 onfront observer 162 will now highly reflect the wrong spectral contentfrom any light that happens to strike the back surface of the filters,substantially degrading stereoscopic image quality and contrast. Evenwhen the reflected light of filter glasses 74 does not directly land onthe back side of the filter glasses 74 for the front observer 162, someof that light will return to the projection screen further decreasingimage quality and contrast for all viewers. While curved filters spreadsthis light out more than flat filters, much of the light will still landon the screen.

FIGS. 21A and 21B illustrate filter glasses 200 having a modified designto mitigate the degradation of image quality due to light reflected fromthe right-eye filter 76R and the left-eye filter 76L according to anembodiment of the present invention. The side view of FIG. 21A andperspective view of FIG. 21B show filter glasses 200 that are configuredto reduce image degradation due to back reflection by redirectingreflected light at a skewed angle, upwards with respect to the viewerposition, so that it is directed away from the display surface 72 (FIG.20A) other viewers sitting in front of the wearer of the filter glasses200. A frame 210 including rims 215 dispose the right-eye filter 76R andthe left-eye filter 76L at a tilt angle θ relative to vertical, so thatreflected light is directed upwards and away from other viewers seatedahead of the wearer of the filter glasses 200.

For typical viewing environments, the tilt angle θ is preferably betweenabout 5 to 20 degrees. A larger tilt angle may be preferred forembodiments where there is a very short distance between the wearer ofthe filter glasses 200 and the display surface 72. An extreme examplewould be an observer sitting approximately one screen height away fromthe display surface at a vertical position approximately ¼ of a screenheight from the bottom. In this case, light from the bottom of thedisplay surface 72 reaches the filter glasses 200 from a direction about14 degrees below the horizontal and light from the top of the displaysurface 72 reaches the filter glasses from a direction about 37 degreesabove the horizontal. Thus the filters would need to be tipped up to atilt angle of approximately 37 degrees in order for all of reflectedlight to be directed over the top of the display surface 72. This levelof angular tilt may not be practical from an aesthetics point of view.Most audience viewers prefer to be at center level or higher with thescreen suggesting a maximum tip of 26 degree would be more practical.Significant benefits can be realized even when the tilt angle θ is lessthan this level since the light from all viewers returning to the screenis additive, therefore any reduction in the stray light provides acorresponding image quality improvement.

For cases where the left-eye filter 76L and the right-eye filter 76Rinclude dichroic filter stacks, the tilting of the filters willgenerally cause the edge transitions in the spectral transmittancecurves to shift as has been discussed earlier. In this case, it may bedesirable to adjust the dichroic filter designs to provide the desiredspectral transmittance characteristics.

In some embodiments, the frame 210 include optional opaque side shields220 that block at least some of the stray light from reaching the rearsurface of the left-eye filter 76L and the right-eye filter 76R. In apreferred embodiment, the rims 215 are made using a moldable materialand the tilt angle θ is provided by appropriately molding the shape ofthe rims 215. In an alternate embodiment illustrated in FIG. 21C, theframe 210 include a hinge mechanism 225 that enables the rims 215 to bepivoted to provide a variable tilt angle θ. In this way, the tilt anglecan be adjusted as appropriate for the viewing environment.

In the illustrated embodiments, the front and back surfaces of theleft-eye filter 76L and the right-eye filter 76R are shown to besubstantially planar and behave as flat plates. In other embodiments,the left-eye filter 76L and the right-eye filter 76R may be provided ascurved plates with spherical or aspherical curved surfaces. In thiscase, the tilt angle is defined relative to a best fit plane through thecurved surfaces.

FIGS. 22 and 23 shows filter glasses 200 worn by rear observer 160 andfront observer 162, according to an embodiment of the present invention.The rims 215 in the filter glasses 200 are arranged to orient theleft-eye filter 76L and the right-eye filter 76R at an appropriate tiltangle so that reflected light 235 produced when incoming light 230 fromthe display surface 72 (not shown in FIG. 22) is reflected from theleft-eye filter 76L and the right-eye filter 76R of the filter glasses200 worn by the rear observer 160 is directed over the heads of otherobservers (e.g., front observer 162). As a result, the reflected light235 from the filter glasses 200 for the rear observer 160 is less likelyto negatively impact the image quality seen by the front observer 162.Preferably, the reflected light 235 is directed over the top of thedisplay surface 72 so that it does not add flare light to the displayedimage.

In an alternate embodiment of the present invention, there is provided astereoscopic imaging apparatus that uses one or more tunable lightsources to provide the different spectral bands in at least one of thecolor channels. Referring to FIG. 24, there is shown a schematic diagramof a red imaging channel 140 r that has a red tunable light emitter 152r, such as a tunable narrow-band, solid-state laser, for example. Thered tunable light emitter 152 r can selectively provide light in atleast two different states. In the first state, the red tunable lightemitter 152 r provides light in the R1 spectral band that is used toform the right-eye image, and in the second state the red tunable lightemitter 152 r provides light in the R2 spectral bands that is used toform the left-eye image. As shown in timing chart 154, the controllersystem 80 is adapted to control the red tunable light emitter 152 r sothat it alternately emits light in the R1 and R2 spectral bandsaccording to a defined temporal sequence. In order to switch withoutbeing detectable to the viewer, the red tunable light emitter 152 r mustbe capable of switching between the color states at a high rate, such asat about 60 Hz, for example.

The emitted light is conditioned by optical components (e.g.,uniformizing optics 44 and one or more lenses 48) to illuminate spatiallight modulator 60. The pixels of spatial light modulator 60 aresynchronously controlled by the controller system 80 according to imagedata for the corresponding right-eye or left-eye image. The resultingimage is then projected to display surface 72 using projection optics 70as described previously.

As illustrated in FIG. 25, the red tunable light emitter 152 r of FIG.24 can be combined with a green tunable light emitter 152 g and a bluetunable light emitter 152 b that provide right-eye and left-eye imagecontent in the blue and green color channels, respectively, to formcolor stereoscopic imaging system 150, having red imaging channel 140 r,green imaging channel 140 g and blue imaging channel 140 b. Each tunablelight emitter emits light in at least two different spectral bands,typically of the same primary color (red, green, or blue). In thisconfiguration, the projection optics 70 can include a beam combiningsystem, such as the dichroic combiner 82 described with reference toFIG. 9, for example.

It can be appreciated that the stereoscopic imaging system 150 whichuses tunable light emitters has advantages over other types ofwavelength-based stereoscopic imaging systems that require multiplelight sources or require multiple banks of filters for filtering lightfrom a single polychromatic (white) light source. For example, theconfiguration described relative to FIG. 5, requires six different lightemitters rather than the three light emitters of FIG. 25. Furthermore,the configuration of FIG. 5 also requires three beam scanners 50 toswitch between the two color states.

Another useful feature of some types of tunable light emitters is thatthey can be used to provide some amount of wavelength “jitter” about acentral wavelength either through creation of multiple simultaneousmodes, high frequency mode hopping or higher frequency tuning around thecentral spectral band, so that the emitted light varies at each momentwith respect to wavelength. In this case, when the controller system 80controls the tunable light emitters to operate in their first state thetunable light emitters can be configured to sequentially emit lighthaving two or more different peak wavelengths within a first spectralband, and when the tunable light emitters to operate in their secondstate the tunable light emitters can be configured to sequentially emitlight having two or more different peak wavelengths within a secondspectral band that is spectrally adjacent to the first spectral band.Randomness of the spectral output within the wavelength range of thespectral band reduces undesirable effects of highly coherent light, suchas speckle, common to many types of laser projection systems.

The red, green and blue tunable light emitters 152 r, 152 g and 152 b ofFIG. 25 can be any type of tunable light source known in the art. Insome embodiments, the tunable light emitters are solid-state lightsources, such as tunable light-emitting diodes (LEDs) or tunable lasers.Tunable lasers change emitted output wavelength using one of a number ofdifferent possible mechanisms. One such approach involves the control ofan optical cavity using micro-electromechanical systems (MEMS) devicescapable of rapidly switching between mechanical states as described inthe article “760 kHz OCT scanning possible with MEMS-tunable VCSEL” byOverton (Laser Focus World, p. 15, July 2011). In the described device,an electrostatically actuated dielectric mirror is suspended over thetop of a laser structure in order to adjust the wavelength.

An alternate approach to providing a suitable tunable laser is to use abistable laser. Feng et al., in an article entitled “Wavelengthbistability and switching in two-section quantum-dot diode lasers” (IEEEJournal of Quantum Electronics, Vol. 46, pp. 951-958, 2010), disclosethe use of two-section mode-locked quantum dot lasers that switch indiscrete integer multiples in 50 picoseconds. The operation of thisdevice is based on the interplay of the cross-saturation and selfsaturation properties in gain and absorber and the quantum-confinedStark effect in absorber. This type of laser can be easily tuned byvarying a current injection level or a voltage level.

A type of tunable LED that can be used in accordance with the presentinvention is described by Hong, et al. in an article entitled“Visible-Color-Tunable Light-Emitting Diodes,” Advanced Materials, Vol.23, pp. 3284-3288 (2011). These devices are based on gallium nitridenanorods coated with layers of indium gallium nitride to form quantumwells. The thicknesses of the layers vary naturally when they areproduced and, by changing the applied voltage, current can be pushedthrough different layers, thereby providing different colors of emittedlight.

The invention has been described in detail with particular reference tocertain preferred embodiments thereof, but it will be understood thatvariations and modifications can be effected within the spirit and scopeof the invention. For example, the light emitters used in the variousembodiments can be of any type known in the art, and can include arraysof lasers or other emissive devices combined onto the same optical axisusing prisms or other combining optics.

Optical systems, typically represented by a lens or a block in theschematic drawings provided, could include any number of opticalcomponents needed to guide and condition the illumination or imagedlight.

The spatial light modulator 60 in each color channel can be any of anumber of different types of spatial light modulator, such as a liquidcrystal array or a Digital Light Processor available from TexasInstruments, Dallas, Tex. (a type of digital micro-mirror array) forexample.

In some embodiments, a color channel can have two spatial lightmodulators, one corresponding to each eye of the observer, so that thereare six spatial light modulators in a stereoscopic digital projectionsystem. Alternately, each color channel can have a single spatial lightmodulator as in FIG. 5, shared between left-eye and right-eye imagecontent using color scrolling or some other resource-sharing method,such as alternately activating the different spectral bands according toa timing pattern.

In some embodiments, additional filtering can be provided in theillumination path to attenuate the spectral content from one or more ofthe light emitters so that the adjacent spectral bands are substantiallynon-overlapping.

While the invention has been described with reference to a stereoscopicdigital projection system which projects images onto a display screen,it will be obvious to one skilled in the art that the invention can alsobe applied to other types of stereoscopic digital display systems thatdo not involve projection. For example, stereoscopic digital soft-copydisplays can be used to directly form the left-eye and right-eyestereoscopic images on a display surface. The soft-copy display can useany type of display technology known in the art such as LED displays andLCD displays.

PARTS LIST

-   12 light emitter-   12L left-eye light emitters-   12R right-eye light emitters-   18 optics-   20 spatial light modulator-   28 a image frame-   28 b image frame-   28 c image frame-   28 d image frame-   28 e image frame-   30 light redirecting prism-   32 image region-   34 b band of light-   34 g band of light-   34 r band of light-   35 b band of light-   35 g band of light-   35 r band of light-   36 a band of light-   36 b band of light-   38 image frame-   38 a image frame-   38 b image frame-   38 c image frame-   38 d image frame-   38 e image frame-   40 r red color channel-   40 g green color channel-   40 b blue color channel-   41L left-eye image forming system-   41R right-eye image forming system-   42 a light source-   42 b light source-   43L light source-   43R light source-   44 uniformizing optics-   46 beam combiner-   48 lens-   50 beam scanner-   52 prism-   54 lenslet array-   56 block-   58 integrating bar-   60 spatial light modulator-   60L spatial light modulator-   60R spatial light modulator-   62 frame-   66F front surface-   66R rear surface-   68 dichroic surface-   70 projection optics-   72 display surface-   74 filter glasses-   75 overlap region-   76L left-eye filter-   76R right-eye filter-   77B bandpass filter transmission band-   77E edge filter transmission band-   78L left-eye filter transmittance-   78R right-eye filter transmittance-   79L left-eye filter transmittance-   79R right-eye filter transmittance-   80 controller system-   82 dichroic combiner-   84 dichroic surface-   86 dichroic filter stack-   87 absorptive filter layer-   88 substrate-   90 illumination optics-   92 beam scanning optics-   94 first stage-   96 second stage-   100 stereoscopic digital projection system-   110 stereoscopic digital projection system-   120 projector apparatus-   130 graph-   135 graph-   140 b blue imaging channel-   140 g green imaging channel-   140 r red imaging channel-   150 stereoscopic imaging system-   152 b blue tunable light emitter-   152 g green tunable light emitter-   152 r red tunable light emitter-   154 timing chart-   160 rear observer-   162 front observer-   170R right-eye dichroic filter transmittance-   171R right-eye dichroic filter reflectance-   172R right-eye absorptive filter transmittance-   173R right-eye hybrid filter transmittance-   174R right-eye hybrid filter reflectance-   175R transmitted right-eye light-   175L transmitted left-eye light-   176R reflected right-eye light-   176L reflected left-eye light-   186L left-eye incident light-   187L left-eye reflected light-   194 right eye-   196R right-eye incident light-   196L left-eye incident light-   197L left-eye reflected light-   198R right-eye transmitted light-   200 filter glasses-   210 frame-   215 rims-   220 side shield-   225 hinge mechanism-   230 incoming light-   235 reflected light-   A1 area-   A2 area-   B spectral band-   B1 spectral band-   B2 spectral band-   G spectral band-   G1 spectral band-   G2 spectral band-   O axis-   R spectral band-   R1 spectral band-   R2 spectral band-   S wavelength separation-   θ tilt angle-   θ1 angle-   θ2 angle

1. A stereoscopic digital projection system that projects a stereoscopicimage including a first-eye image and a second-eye image onto a displaysurface, comprising: one or more narrow-band, solid-state, tunable lightemitters, each controllable to alternately provide emitted light in afirst state having a corresponding first spectral band with a respectivefirst central wavelength and emitted light in a second state having acorresponding second spectral band with a respective second centralwavelength, the first and second spectral bands being substantiallynon-overlapping; an image forming system including at least one spatiallight modulator for forming modulated images by modulating light fromthe tunable lights emitters, each spatial light modulator having anarray of controllable spatial light modulator pixels; a controller thatsynchronously controls the state of the tunable light emitters and thespatial light modulator pixels, wherein the spatial light modulatorpixels are controlled responsive to image data for the first-eye imagewhen the tunable light emitters are in the first state thereby providinga first-eye modulated image and the spatial light modulator pixels arecontrolled responsive to image data for the second-eye image when thetunable light emitters are in the second state thereby providing asecond-eye modulated image; and projection optics for delivering thefirst-eye modulated image and the second-eye modulated image to adisplay surface.
 2. The stereoscopic digital projection system of claim1 wherein at least one of the tunable light emitters is a tunable laser.3. The stereoscopic digital projection system of claim 2 wherein thetunable laser includes a MEMS tunable control means.
 4. The stereoscopicdigital projection system of claim 2 wherein the tunable laser includesquantum dots.
 5. The stereoscopic digital projection system of claim 2wherein the tunable laser is a bistable laser.
 6. The stereoscopicdigital projection system of claim 5 wherein the bistable tunable laseris tuned by adjusting a current injection level or a voltage level. 7.The stereoscopic digital projection system of claim 1 wherein at leastone of the tunable light emitters is a tunable LED.
 8. The stereoscopicdigital projection system of claim 1 wherein when the tunable lightemitters are controlled to provide emitted light in the first state, thetunable light emitters are further controlled to vary the wavelength ofthe emitted light within the respective first spectral band, and whenthe tunable light emitter is controlled to provide emitted light in thesecond state, the tunable light emitter is further controlled to varythe wavelength of the emitted light within the respective secondspectral band, thereby providing speckle reduction.
 9. The stereoscopicdigital projection system of claim 1 wherein the first spectral band andthe second spectral band each have full-width half-maximum bandwidths ofless than 15 nm.
 10. The stereoscopic digital projection system of claim1 wherein the first spectral band and the second spectral band both fallwithin either a red color spectrum, a green color spectrum or a bluecolor spectrum.
 11. The stereoscopic digital projection system of claim1 wherein the narrow-band, solid-state, tunable light emitters includes:a red tunable light emitter tunable controllable to alternately provideemitted light in a first state having a corresponding first red spectralband and emitted light in a second state having a corresponding secondred spectral band; a green tunable light emitter tunable controllable toalternately provide emitted light in a first state having acorresponding first green spectral band and emitted light in a secondstate having a corresponding second green spectral band; and a bluetunable light emitter tunable controllable to alternately provideemitted light in a first state having a corresponding first bluespectral band and emitted light in a second state having a correspondingsecond blue spectral band; and wherein the red, green and blue tunablelight emitters are controlled to emit in their respective first statesto provide the first-eye modulated image and the red, green and bluetunable light emitters are controlled to emit in their respective secondstates to provide the second-eye modulated image.
 12. The stereoscopicdigital projection system of claim 11 wherein the image forming systemincludes three spatial light modulators, one associated with each of thered, green and blue tunable light emitters.
 13. The stereoscopic digitalprojection system of claim 12 further including a beam combining systemfor combining light provided by the three spatial light modulators ontoa common optical axis for projection onto the display surface using theprojection optics.
 14. The stereoscopic digital projection system ofclaim 11 wherein a first white balancing operation is used to whitebalance the first-eye image and a second white balancing operationdifferent from the first white balancing operation is used to whitebalance the second-eye image, the first white balancing operation beingadapted for use with emitted light in the red, green and blue first-eyespectral bands and the second white balancing operation being adaptedfor use with emitted light in the red, green and blue second-eyespectral bands.
 15. The stereoscopic digital projection system of claim11 wherein a first color correction transform is used to transformdigital image data associated with the first-eye image and a secondcolor correction transform different from the first color correctiontransform is used to transform digital image data associated with thesecond-eye image, the first color correction transform being adapted foruse with emitted light in the red, green and blue first-eye spectralbands and the second color correction transform being adapted for usewith emitted light in the red, green and blue second-eye spectral bands.16. The stereoscopic digital projection system of claim 1 furtherincluding filter glasses comprising: a first-eye filter that transmitsmore than 50% of the light from the one or more tunable light emitterswhen they are controlled to emit light in the first state and transmitsless than 5% of the light from the one or more tunable light emitterswhen they are controlled to emit light in the second state; a second-eyefilter that transmits more than 50% of the light from the one or moretunable light emitters when they are controlled to emit light in thesecond state and transmits less than 5% of the light from the one ormore tunable light emitters when they are controlled to emit light inthe first state; a frame into which the first-eye filter and thesecond-eye filter are mounted, the frame being adapted to position thefirst-eye filter in front of an observer's first eye and to position thesecond-eye filter in front of the observer's second eye.
 17. Astereoscopic digital display system that displays a stereoscopic imageincluding a first-eye image and a second-eye image on a display surface,comprising: one or more narrow-band, solid-state, tunable lightemitters, each controllable to alternately provide emitted light in afirst state having a corresponding first spectral band with a respectivefirst central wavelength and emitted light in a second state having acorresponding second spectral band with a respective second centralwavelength, the first and second spectral bands being substantiallynon-overlapping; an image display system having an array of controllablepixels for forming displayed images on the display surface by modulatinga light intensity from the tunable lights emitters; a controller thatsynchronously controls the state of the tunable light emitters and thecontrollable pixels, wherein the controllable pixels are controlledresponsive to image data for the first-eye image when the tunable lightemitters are in the first state thereby providing a first-eye modulatedimage and the controllable pixels are controlled responsive to imagedata for the second-eye image when the tunable light emitters are in thesecond state thereby providing a second-eye modulated image.
 18. Thestereoscopic digital display system of claim 19 wherein the stereoscopicdigital display system is a stereoscopic digital projection system. 19.The stereoscopic digital display system of claim 19 wherein thestereoscopic digital display system is a stereoscopic digital soft-copydisplay system.